(1) Field of the Invention
The present invention relates to a copying machine provided with an image reading apparatus for scanning moving original documents.
(2) Related Art
A conventional copying machine of a stationary document scanning type which reads original documents placed on the platen glass by moving the scanner in an automatic paper selection mode (hereinafter simply referred to as "the APS mode") forms images on copying sheets as follows. Firstly, the size of an original document transported to a predetermined position on the platen glass is determined by photoelectric sensors provided below the platen glass. The sheet size is selected according to the determined document size, and copying sheets of the determined sheet size are fed into an image forming unit. The scanner moves in a parallel direction with the platen glass to read the image data from the original document, and an image is finally formed on the copying sheet in accordance with the image data.
In the APS mode, a copying sheet of a suitable size is selected automatically as described above, and thus, image formation on copying sheets of a wrong size can be effectively prevented. For this reason, many types of copying machine today employ the APS mode as a standard mode. Especially in a copying machine of a stationary document scanning type, the feeding of copying sheets of a suitable size can be started almost at the same time that the document reading is started, because the size of copying sheets to be fed is determined prior to the start of the document reading. Thus, the time required for the copying operation, starting from image reading and ending in image formation, can be shortened.
With such a copying machine, however, the scanner needs to be returned to the home position every time it reads an original document, which will result in low processing speed when making a plurality of copies. Accordingly, the overall copying operation will be adversely affected and become slow.
For a solution, an increasing number of copying machines provided with an image reader unit of a moving document scanning type have been used in recent years. In a copying machine of this type, the scanner is fixed to the document reading position, and image data are read by moving original documents in the sub-scan direction at the same speed as the scan speed of the copying machine of a stationary document scanning type. Here, the original documents are fed and transported by an automatic document feeder.
This scanning method has the advantage that the document reading is performed at very high speed, because a plurality of original documents are transported to the reading position one by one and discharged onto a discharge tray without stopping for image reading.
When setting the APS mode in a copying machine of this moving document scanning type, however, the image data of one page original document are first read and written into the memory, and the document size is then determined based on the image data written in the memory. After that, the copying sheet size is determined and the sheet feeding is started. As a result, image formation cannot be performed in parallel with image reading, and the whole meaning of employing the moving document scanning method, which is aimed at improving the reading speed, will be spoiled.
To avoid such an undesirable situation, the document feed tray may be provided with photoelectric sensors for detecting original documents to determine the document size by detecting the document both in longitudinal and transverse directions. When making copies from original documents of various sizes, however, there is a problem that only the largest size document can be detected by the sensors.
For an alternative solution, the document size may be determined while an original document is transported from the document feed tray to the document reading position by the automatic document feeder. Here, the original document passing through is detected by photoelectric sensors or the like, and the document length in the transport direction is determined from the elapsed time and the transport speed. Based on the combination of the document length and a detection signal in the transverse direction, the document size can be determined prior to the start of image reading.
According to the above document size determination method, image formation can be performed in parallel with image reading, because the document size is determined prior to the start of the image reading, as in the stationary document scanning method.
To detect an original document passing through, however, the top end and the bottom end of the original document being transported should be precisely sensed by a photoelectric sensor and the like. Therefore, the photoelectric sensor should be disposed past the separating position, where original documents are separated from one another by separating rollers.
In order to determine the document size prior to the start of image reading, the length from the separating rollers to the document reading position should be longer than the transport-direction length of an original document of the largest readable size. If the largest readable size of document is A3 in the longitudinal direction (hereinafter referred to as A3T, T indicating "longitudinal"), the length of the transport path should be at least 420 mm.
As a result, the automatic document feeder will become large, taking up too much space. The automatic document feeder of this type has a further disadvantage of being heavy, which makes it very difficult for users to lift it up and place an original document directly on the platen glass.
This kind of problem cannot be avoided yet in a copying machine of a moving document scanning type which employs an automatic magnification selection mode (hereinafter simply referred to as "AMS mode"), as long as the document size has to be determined as a preparation for image formation. The AMS mode is set to determine such magnification that the image of an original document can be printed on a copying sheet of a predetermined size, and to subject the image data to magnification change to form the image at the determined magnification.